Transmission control in two-way signaling systems



-May 14, 1935.

B. G. BJoRNsoN .Tl 3.9m

/NvE/vron B. G. BJORNSON @y J May 14, 1935. B. G. BJoRNsoYN 2,001,088

TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEMS .V

Filed Jan. 27, 1932 s sheets-sheet l HANGOVER 6/ r-ar N, cmcu/r /M/ENTOR J a. c. JoR/vso/v HVJ.

A TTORNEV May 14, 1935. B.' G. BJoRNsoN TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEMS Filed Jan. 27, 1932 5 sheetshaet s /NVE/VTOR B. G. vBJORNSN By AHvNN m. GP* l A 7' TORNE y Patented May 14, 1935 TRANSMISSIGN CONTRUL IN TWO`WAY `SIGNALING SYSTEMS Bjorn i Gr.` Bjornson, Brooklyn, N. Y., vassignor to Bell '.lelephone` Laboratories,

Incorporated,

New York, Ni Y., a corporation of New York i ApplicationJanuary 27, 1932, Serial No. 589,197

8 (liaisons.` (Cl. 179-170) 'loy obtainproper operation of long two-way Y signaling systems, forl example,` long four-wire voice frequency orwcarrier` telephone circuits, ithas been found necessary'inthe past to uti- 151" 1ize-- inI f connection therewith signal-controlled apparatus for `effectively disabling one of the associated two-wire paths while signal transmission is taking place over the other, so as to prevent echo currents or reflected currents from ZOnbeing transmitted back tothe transmitting end of-` the system and.v causing a disturbance or singing. The disabling apparatus usually com-` prises means, such as anamplier-rectier controlcircuit; for divertingya portion of the sig- 25g=nalinggcurrent from one-path andl utilizing it to control 4the open-circuiting orshort-circuiting of the oppositelydirected path, or to control the'operation characteristicsof an amplifier in the oppositely directed path. 30u In certain circuits of the prior art, these signal-'controlled devices, orecho Suppressors as they are commonly called, one'oifv which is-associated withl each two-wirepath, are located at some intermediate; station,` for example, at a itmidway point. Such an varrangement has the disadvantagei that it may prevent the subscriber who starts to talk in one direction from getting immediate controlV of the circuit` after it has been used-by the subscriber talking in the other l10i-direction. This is due to the-fact that, owing to the time intervalwhich it takes `for a wave to be transmitted from the-echo suppressor associated., with onev side of the circuit through the terminal to the point at which the opposite- 45, ly-directed circuit is disabled, the echo suppresH sor must be provided with'hang-over means to l maintain the 'line disabled *forr a certain length of time after the rst talker has finished talking. In otherV circuits of the prior art this 501' difficulty has been` eliminated-"by locating the two units of thefecho suppressor respectively at or Anear the endsof` the four-wire circuit. The

latter arrangement, thefso-calledterminal echo suppressor, enables? asubstantial reduction in 55 the breakingttime cuits of the prior art the terminal echo supeA pressors have been of the receiver type, the` suppression unit at each terminal comprisinga suppressor responsive to signals in the incoming two-wire circuitJ to disable the outgoing two-wire circuit and means responsive to signals in the,

`outgoing two-Wire circuit to disable the sup:

pressor at the same terminal.

One embodiment of the present invention relates to a two-way signaling system employing terminal echo Suppressors ofwhat may be called the sender type. In this system the suppres-` sion device at each terminal comprises a, suppresser responsive to signals in the outgoing two-Wire circuit to disable theincoming twowire circuit, and means responsive to signals.A in the incoming circuit to block the suppressor at the same terminal.

A feature -of the invention is the provision in a two-way signaling system employing terminal echo Suppressors and automatic volume control circuits, of switching means for preventing :falsev operation of the volume control circuits by vthe Suppressors.

In a modification of the invention particularly applicable to a two-way radio or cableA telephone circuit, Vthe echo suppression apparatus oomprises an echo suppressor unit of the sender type and an echo suppressor unit oi the receiver type both located at the same, terminal.

The exact nature and the advantages of, the circuits of the invention will be clearfrom the following detailed description thereof when read in connection with the accompanying drawings in which:

Fig. 1 shows schematically one four--wire` channel of a multiplex carrier cable telephonesystem vemploying the sender type of echo suppressor embodying the invention;

Fig. 2 shows schematically one four-Wire Nchannel of a `multiplex carrier cable telephone` system employing the receiver type of terminal echo suppressor; and

Fig. 3 shows schematically a long four-Wire toll telephone circuit employing a sender type` oiA terminal echo suppressor in connectionwith-4 one side of the four-wire circuitand a receiver type of terminal echo suppressor in Vconnection with the other side of the four-wire icircuit. l

Fig. i` shows4 schematically a multiplex, four` wire carrier cable system for transmittingltelephonic` signals in either directionvSbetween a two-way telephone circuit Lrincoming at a1 west j station` A and a` two-way telephone circuit La.,

incoming at an east station B. Only one carrier channel for each direction has been illustrated in detail.

The west tc east, one-way transmission path EA forming one carrier channel between stations A and B includes at station A a volume control device l, a modulator M1 and associated carrier frequency oscillator O1, and a band pass filter 2, and at station B a band pass iilter 3, a demodulator DM1 and associated carrier frequency oscillator` O2 and a compensating volume control 4. The east to West, one-way transmission path WA forming the second carrier channel connecting stations B and A includes at station B a volume control device il, a modulator M2 and associated carrier frequency oscillator O3 and band pass filter 6 and at station A a band pass nlter 7, a demodulator DMZ and associated carrier frequency oscillator O4, and a compensating volume control device 3. The input of the west to east transmission path EA and the output of the east to west transmission path WA are connected in substantially conjugatc relation with each other and in energy transmitting relation with the east end of the telephone circuit L1 in well-known manner by means of the hybrid coil H1 and associated balancing network N1. Similarly, the input of the east to west transmission path WA and the output of the west to east transmission path EA are connected in substantially conjugate relation with each other and in energy transmitting relation with the west end of the telephone circuit L2 in well-known manner by means of the hybrid coil H2 and associated balancing network N2. In addition to the apparatus mentioned the west to east transmission path EA and the east to west transmission path WA would contain a number of one-way amplifying devices located at various points therealong for giving the required degree of amplication to the speech frequency signals or the signal modulated carrier as determined by the attenuation characteristics of these paths between the stations A and B;

The volume control device I in the transmission path EA at station A, and the volume control device 5 in the transmission path WA at station B, are utilized to maintain the volume level of the telephonie currents transmitted to modulators M1 and M2 respectively constant within denite limits irrespective of a wide range of volumes in the source of signals. This is desirable to insure proper signal volume at the receiving end of the system and to insure proper operation of the echo Suppressors associated with the transmission path, to be described. The compensating volume control device B in the output of the transmission path WA a1; station A and the corresponding compensating volume control device 4 in the output of the transmission path EA at station B, are utilized to make an adjustment in the volume level in these paths which is the inverse of that produced in the path transmitting signals, so that the transmission equivalent over the loop cir cuit comprising both paths EA and WA is not changed by the volume adjustments. These volume control adjustments are preferably made automatically under control of the signaling currents as indicated schematically in Fig. l where the volume control device i and the compensating volume control device 3 are controlled by operation of a control circuit 9 which in turn is controlled by the signaling current diverted from the path EA by the forward-acting .control circuit I0 connected to the input of the device l and the backward-acting control cir cuit Il connected to the output of the control device I, and where the volume control device 5 and the compensating volume control device 4 at station B are controlled by control circuit l2 which is controlled in turn by signaling current diverted from the path WA by the control circuits i3 and i4 respectively connected to the input and output of the volume control device 5 in the path WA. These automatic volume control systems, for example, may be of the type disclosed in the Patent No. 1,853,070 issued to D. Mitchell, April 19, 1930, and Patent No. 1,853,974 issued to I-Iogg et al, April 12, 1932, in which three-electrode space discharge amplifying devices are utilized as loss elements in the signal transmission paths and the potentials on the grids of these devices are automatically controlled by adjusting the charge on condensers in the grid circuits, in accordance with the volume level of the signals in said paths to adjust the gain of the space discharge devices in proper manner.

The modulators M1 and M2, and the demodulators DM1 and DMz may be of any of the wellknown types, for example, Vacuum tube modulators and demodulators as disclosed in Carson Patent No. 1,343,308, issued June 15, 1920. The modulators are supplied with carrier currents of a given frequency from the associated oscillators O1 and Os, which may be of any suitable character, and are so arranged that nor mally the carrier frequency is suppressed, but when the band of modulating current frequen* cies, that is, the signaling current received from the telephone circuit L1 or L2, is supplied to the modulator the carrier frequency is modulated by the modulator so that a double band of frequencies appears in the output circuit of the modulator. The band filters 2 and 6 in the output of modulators M1 and M2 respectively, are preferably of the general type disclosed in U. S. patents to George A. Campbell No. 1,227,113 and No. 1,227,114, issued May 22, 1917. These filters may be so designed as to transmit one of the side bands, preferably the upper, and to suppress the other, so that the modulator in.

conjunction with the band filter, operates to step up the band of speech frequencies supplied to the modulator to a band of equal width, but of carrier frequencies. The band filterf in the input of the demodulator DM1 at station B in the transmission path EA is similar to the band pass filter 2 at station A, and may be arranged to transmit the same band of frequencies. Similarly, the band pass filter lat station A in the transmission path WA is similar to the band pass lter 6 at station B in the same path, and may be arranged to transmit the same band of frequencies.

The demodulator DM1 at station B in the transmission path EA, and the demodulator DMz at station A in the transmission path WA, which are preferably vacuum tube demodulators of the type disclosed in the above mentioned Carson patent, operate when supplied with carrier current from the associated carrier frequency oscillator O2 or O4 and with the band of modulated signaling current from the output of the band pass lter 3 or 1, to combine these frequencies so as to produce in the output of each demodulator a band of signaling frequencies corresponding to the band of signaling frequencies impressedfupon the modulator at the transmitting end of the system.

A terminal echo suppressor of the sender type is associated with the four-wire cable cir-A `and another portion, which` may be referred to as the interlocker, which is responsive to sig- `nais in the incoming` two-wire circuit to disable the suppressor portion of the unit. Thus, the suppressor portion of the sender type terminal echo suppressor at station A, illustrated `in Fig.`

l, comprises a control circuit i5 having its input connected across the path EA in the input of the modulator M1 therein, and comprising a delay circuit it, a signal-controlled, relay- 20` controlling device il, which may be a vacuum tube, amplifier-rectiiier circuit` of the type wellknown in the art, and the winding of a mechanical relay i8 connected across the output of the device Il'. The normally closed switch contacts i9 in the transmission path WA in the output of the demcdulator DMZ therein are adapted to be opened by operation of the relay i8, so as to disable the output of the east to West transmission path WA.

The interloclrer portion of the sender type terminal echo suppressor at station A cornprises a control circuit 2t having its input connected across the east to west transmission path WA. between the disablingpoint therein and the input of the compensating volume control def vicet, and comprising a delay circuit 2i, a signal-controlled, relay-controlling device Z2 similar to the device [Land thewindings of mechanical relays 23, 2t and 25 connected in parallel across the output of the device t2. The normally closed switch contacts 25 in the output of the control circuit iii are adapted to be opened in response to operation of the mechanical relay 23, to disable the mechanical relay it' in the suppressor circuit.

The normally closed switch contacts ill and 2li in the circuits connecting the control circuit il of the automatic volume control system to the volume control device i in the path EA and the compensating volume control device 8 in the path WA, re-

spectively, are adapted to beiopened in response to operation of relays 2li and 25, to prevent false operation of the automatic volume control system at station A in response to current in the transmission path EA when signal 'transmission is taking place in the direction from `east to west Yover the transmission path WA.

Similarly, the "sender type of terminal echo suppressor located at station B comprises a suppressor portion and an interlccker portion, The suppressor portion comprises a control circuit 29 having its input connected across the east to west transmission path WA in the input of the modulator M2 therein, and comprising a delay circuit Bil, a signal-controlled, relay-controlling deviceli, similar to the devices il and 22, and the winding of mechanical relay 32 connected across the output of the device 3l. The normally closed switch contacts 33 in path EAr locatedbetween the output of the demodulator DlVli and the compensating volume control device@ therein are adapted to be opened by operation of the mechanical relay 32 when its winding is energized; in response to operation` of the control device 3i, to disable the output ofthe transmission path EA.

The interlocker portionof `the -sender ,type terminal echo suppressor located at station B comprises a controlcircuit 3ft having its input i connected across the path EA between the disabling point therein and the compensating volurne control device l and comprising a delay circuit 35, a signal-controlled, relayfcontrolling device similar to the devices ll, 22 and lli, and the windings of the mechanical relays 3l, tiand Si! connected in parallel to the output ci the device 3b. The normally closed switch contacts iii in the output of the` control circuit 29 are adapted tobe opened by operation of the relay El in response to energization of its winding by operation of thecontrol device 36, to dis-v able the input of the mechanical relay 322 in the suppressor portion of the terminal echo supressor unit at station B. The normally closed switch contacts il and @i2 in the circuits connecting` the controlcircuit i2 of the automaticY volume control system at station B to the volurne control device 5 in the path WA and the compensatingvolume control device in the path EA, respectively, are adapted to be opened in response to operation of relays 38 and 35i, to prevent false operation or" the automatic volume control system in response to current in the path WA whenI signals are being transmitted over the west to east transmission path EA.

As indicated in the drawing, the system of Fig. l mayrcoinprise other carrier channels C1, C2 for transmitting other conversations in the direction from west to east and other carrier channels C3, C4 for transmitting other conversations in the direction from east to west, between the telephone lines L1 and L2. These additional carrier channels will contain apparatus identical with that inthe channels which have been illustrated in detail except that the carrier oscillator for each channel will be designed to generate a different carrier frequency requiring that the band filters associated with the modun lator and deniodulatcr in each channel have dierent transmission ranges than the similar iilters in the other channels, this range being determined by the particular carrier frequency assigned to that channel.

The delay circuits iii and 2i at station A and the delay circuits .it and 35 at `station B may be networks or any type suitable for producing a delay in the transmission therethrough of al ternating current waves impressed on their inputs, for example they may be low pass electrical iilters such as `disclosed in U. S. patents to Campbell, Nos. 1,227,113 and l,22"1,114l, issued Nay 22, l9l', or networks for a similar purpose such as disclosed `in Arnold Patent No. 1,565,302, issued December l5, 1925. The delay circuit lli in the control path E5 at station A, and the delay circuit 3E? in the control circuit 2li at station B are utilized to provide a given amount of delay in the operation ci the suppressor Vcircuit at the terminal in response to the speech waves in the transmitting path thereat, to accomplish the disabling of the receiving path at the same terminal staion. This delay is preferably made substantially equal to T where T is the overall transmission time of one one-way path voi the four-wire circuit between the terminal stations in order that the two-Wire circuit over which transmission ofsignals is not taking place may be disabled `substantially at the same time as the signals transmitted over theother two-wire path seize control of the switching circuit at the receiving terminal, although, as will be brought out later, under certain conditions this delay may be somewhat different. These delays in the operation of the suppressors might be obtained also by making the suppressor relays I8 and 32 slow operating by suit-able mechanical design, in which case the delay circuits I6 and 33 would not be required.

In the case where the delay time in operation of the suppressor is made equal to T, which is preferable, substantially no time delay in the operation of the interlockers, i. e. the control circuits 2B and 3H, will be required. Hence the delay circuit 2i in control circuit 20 and the delay circuit 35 in control circuit 34 may be omitted. Howe-Ver, where the delay times of the delay circuits iii and 35 in the suppressor circuits at the respective terminal stations are greater than T, which will be the case in an alternative method of operation to be described below, the delay circuits 2! and 35, or a suitable design or" the interlocker relays 23 and 3l to make them suiciently slow operating, will be required to give the proper time delays in the operation of the inter-lockers.

In the preferred case, the control circuit I5 controlling the disabling of the transmission path WA, and the control circuit 29 controlling the disabling of the transmission path EA should be designed to have suitable hang-over times in their operation so as to remain operated to maintain the controlled transmission path disabled a given time after the supply oi' speech current to the control device Il in control circuit !5, or to the control device 3! in control circuit 29 ceases. These hang-over times in the operation of the eche Suppressors may be obtained by suitable mechanical design of the mechanical relays iii and 32 to make them slow releasing to the required degree, or by any other suitable means. In the preferable case, the hang-over time of each of these relays will be made equal to T, although, as will be brought out later, under certain conditions, these hangover times may be made less than T. Preferably, the interloclier circuits, i. e. control circuit 2Q at station A and control circuit 34 at station i3 are designed to have suitable hang-overs in their operation to prevent the echo Suppressors and the automatic volume control circuits at the terminal stations from being prematurely made operative after they have been disabled, when the supply of speech waves to control circuits and 3ft respectively, ceases. As in the case ci the echo suppressor circuits, these hang-over times in the operation oi the interlockers may be produced by suitable design of the mechanical relays 23, and '25 at station A and the mechanical relays 3l', 33 and 39 at station B to make them slow releasing to the required degree. These hang-over times in the preferable case are made equal to T, although, as will be brought out later, under certain conditions these hangover times will be made larger than T.

The operation or" the system of Fig. l will now be described. It will be assumed that speech waves for transmission from west to east are being received over the two-wire circuit L1, and, at that time, no speech waves for transmission from east to west are being received at station B over the two-wire circuit L2.

The speech waves will be impressed by the hybrid coil H1 upon the input of the transmission path EA and transmitted thereover toward the modulator Mi. A portion' of thes speech Waves is diverted from the path EA in the input of the volume control device I by control circuit It, and another portion is diverted from the path EA in the output of the volume control device I by control circuit II, these two portions being impressed upon the control circuit 9 causing its operation in suitable manner to adjust the volume control device I in the path EA so as to maintain the speech waves in its output at a substantially constant Volume level, and to adjust the compensating volume control device 3 in the path WA in inverse manner so as to insure that the transmission equivalent around the loop comprising paths EA and WA is notl changed by the adjustment of the volume control device i. The main portion of the speech waves in the output of the volume control device I then is impressed upon the input of the modulator M1. A small portion, however, oi the speech waves in the output of the volume control device I is diverted into the control circuit and after being delayed in transmission through delay circuit IG for a time T is impressed on the control device I'I causing its operation to energize the winding of the echo suppressor relay i 8 in its output.

Relay i8 will then operate, and will be maintained operated as long as the speech waves are being substantially continuously impressed upon the control device Il and for an additional hang-over interval oi T, to open the normally closed switch contacts I9 in the transmission path WA in the output of the demodulator DMZ therein, thereby rendering that path inoperative thereafter to transmit later received Waves to the input of the interlocker circuit 2!) or to the two-wire circuit L1 from the output of the path WA.

The speech waves impressed upon the input of the modulator Mi in the path EA will meanwhile be combined therein with the carrier frequencies from the oscillator O1 to form combination waves in the output of the modulator. Of these combination waves one band is selected by the band pass ilter 2 and transmitted out over the path EA to the distant station B where it is selected by the band pass filter 3 and impressed upon the input of the demodulator DMi at that station. These Waves are combined in the demodulator DMi with the carrier frequency from the oscillator O2 producing in the output of the demodulator a band of speech frequencies corresponding to the band impressed upon the modulator M1 at station A. The main portion oi this band of speech frequencies will be transmitted through volume control compensating device 4 and will be impressed on the two-way telephone circuit L2 by hybrid coil H2, over which circuit it will be transmitted to the listening subscriber associated therewith. A small portion of the speech waves in the output of the demodulator DMi in the path EA will be diverted into the input of the control circuit 34 and, the delay circuit 35 having been omitted in the case being considered, will cause immediate operation of the control device 36 to energize the windings of mechanical relays 31, 38 and 3Q connected to the output thereof.

Relay 31 will operate immediately to open the normally closed switch contacts 40 in the output of control circuit 29 thereby preventing subsequent operation of the control device 3I in response to speech Waves from the path WA from simultaneously with relay `3l to open the normally closed switch contacts t2 and 4l in the l output circuit of control circuit I2 ofY the` automatic volume control system `thereby preventing subsequent `false operation of that system by Waves subsequently received by path WA from the circuit Lzgor `by unbalance currents` from the path EA transmitted through the hybrid coil H2.

Thejspeech currents ofthe West subscriber associated with the telephone circuit L1 have nowobtained complete control oi the switching `circuits at both stations vA and B.

Now` let itbe supposed that the east subscriber `associated with the telephone circuit L2 starts to talk but slightly after the west subscriber associated with the telephone circuit L1 has started to talk. lt is apparent that the east subscriber will not be able to get his speech current through tothe westsubscriber associated with` L; becausetthe `path WA has been previously open-circuited at station A by the opening of switch contacts t9 in response to the previousoperation of control device il by the liirst talkers speech current.

It will also be apparent that `the eastsubscriber associated with the4 circuit La if hestarts talking after *the west subscriber associated with the circuit L1 will not` be able to seize control of the switching circuit at .station B away -from the rst talker because the cutput of the control circuit ..29 willY have been` disabled by the opening of switch contacts @lli in `response to the first talke cris speech` currentibefcre the second talker can get speech current through to the suppressor relay 32` because ofv the delay or T introduced lby theI delay lcircuitfili in the control circuit 519.

`New let it be supposed that the `west subscriber associatedwith the circuitLi ceases to talk. 'Because `of the delay of 'I' introduced by the delay circuit le in control circuit it at static-n .Agand of the `delayof T in the transmis- 1 sion of the west subscribers speech current over the path to station B, the control device il in `control circuit l5 and the` control device 35 control `circuit .dit atl stationB will remain operatedfor the time .,T after "the first; talkers speech current ceasesA to` arrive at station A `be decnerei over the` circuithl. `At the end of this interval .ci "time control device .il in control` circuit lll and controldevice Slt in control circuit will rele' substantially simultaneously and the winding of echo `:suppressor relay it at stationy A and `the .windings of interlocker relays @ist and Se `at station B lwill consequently ced. ,.lowever, because these relays c are des ned to have a hang-overof T, it will be apparentthat the switching circuits at both terminals will remain in the operated condition initiated by the first tallrers speech current for an additional intervalorfl; Thus, it is evident `that the second tallrer cannot break. in andseize `starts to talk before the West subscriber associated with the circuit L1 Will be'clear `from` the above Vdescription ot the converse condition. y

By changing the amount of delay and' hangover in the operation of the echo suppressor and `interloclrer circuits, the operation of the system a to what happens to the second talkers speech may be changed. For example,` if the delaycircuits l@ and 3S in control circuits Hl andiZSQrespectively, are designed to introduce a delay of 2T instead of T, the delay circuits 2l and 35` in interlocker control circuits 2 and 34, `respec tively, are designed to have a` delay of T instead of O, the hang-over times of echo suppressor relays i3 and in control circuits I5 and 29, respectively, are made O instead of equal` to T, andthe hang-over times of the interlocker relays 23, 2d and 25 in control circuit 2t and interloclzer relays 3l, 38 and 39 in control circuit 34 are made equal to T, then vwith thecircuit inactive, when one party starts talking and the other immediately afterwards the second speakers speech current will partially get through to the rst speaker. 1i the circuit is active and the listener says something immediately on per- .Y

ceiving a pause by the talker, the listeners speech current will partially get through to the talker. i

In still another case the delay circuits l5 and iid in control circuits i5 and 29, respectively,

would be designed so that the echo Suppressors or active, `when one party starts talking and the other starts talking immediately afterwards,

the speech currents of both parties will, get through to the other party, but there will beno echo suppression. c

The sender terminal echo suppressor circuits illustrated in Fig. l are applicable aswell to long tour-wire voice frequency telephone systems as well as to the carrier system as `described. The system for voice frequency operation will be the same as illustrated in Fig. 1 except that the modulators and` associated oscillators and band pass lters will be left out.' The operation would be similar to that just described.

' Fig. 2 shows one carrier channel for each direction of a four-wire carrier telephone, 'circuit employing automatic volume control and receiver terminal echo Suppressors. Except for the echo suppressor circuits, the system of Fig. 2 is identical with that of Fig. 1. 'The apparatus in the identical portions of the systems of Figc 1 and Fig. 2 `bear the same identification characters. l

As inthe case of the sender type of terminal echo suppressor described in` connection With Fig. l, `the receiver type of terminal echo suppressor located at each terminal station A and B of the system of Fig. 1, comprises a suppressor portion and an interloclrer portion.

The suppressor portion of the receiver type terminal echo suppressor located at Ystation A in the. system of `l'fig. 2 comprises a control circuits it having itsinput connected across the .40 `the latter case, whether the circuit is inactive therein, and comprising a signal-controlled, relay-controlling device 44, similar to the device I1 in the system of Fig. l, and the windings of mechanical relays and 46 connected in parallel to the output of the device 44. The interlocker portion of the receiver type terminal echo suppressor at station A comprises a control circuit 41 having its input connected across the path EA in the input of the modulator M1 and comprising a delay circuit 48, a signal-controlled, relay-controlling device 49,

similar to the device 44, and the winding of a mechanical relay connected across the output of the device 49. The normally closed switch contacts 5I in the input of the circuit EA are adapted to bc opened by operation of echo suppressor relay 45 in response to energization of its winding by operation of the control device 44 in response to signaling current impressed upon the input of the control circuit 43 from the path WA, thereby disabling the input of the path EA. The normally closed switch contacts 52 in the output of the control circuit 41 are adapted to be opened by operation of the relay 46 also in response to the operation of the con trol device 44, thereby preventing false operation of the relay 56 in response to subsequent operation of the control device 49 in control circuit 41. The normally closed switch contacts 53 in the output of the control circuit 43 are adapted to be opened by the mechanical relay 5l] when its winding is energized by operation or" the con trol device 49 in response to signals impressed upon the input of the control circuit 41 from the path EA.

The suppressor portion of the receiver type terminal echo suppressor located at station B comprises a control circuit 54 having its input connected across the path EA in the output of the demodulator DM1 therein and comprising a signal-controlled, relay-controlling device 55, similar to the device 44, and windings of mechanical relays 51 and 56 connected in parallel across the output of the device 54. The interlocker portion of the echo suppressor at station B comprises a control circuit 58 having its input connected across the path WA at a point therein between the output of the volume control device 5 and the input of the modulator M2, and comprising a delay circuit 59, a signalcontrolled, relay-controlling device 60, and the winding of a mechanical relay 6I connected to the output of the device 60.

The normally closed switch contacts 62 located in the input of the path WA between the hybrid coil H2 and the input of the volume control device 5 therein are adapted to be opened by the relay 56 when its winding is energized by operation of the control device 55 in response to signal Waves impressed upon the input of the control circuit 54.

The normally closed switch contacts 63 in the output of the control circuit 58 are adapted to be opened by mechanical relay 51 when its Winding is energized by operation of the control device 55 in response to signaling waves impressed upon the input of the control circuit 54 from the path EA, thereby preventing false operation of relay 5I in response to subsequent operation of control device 54 by waves received from the path WA.

The normally closed switch contacts 54 in the output of the control circuit 54 are adapted to be opened by mechanical relay 6I when its windpath WA in the output of the demodulator DMz ing is energized by operation of the control device 55 in response to signals impressed upon the input of the control circuit 58 from the path WA, thereby preventing false operation of the relays 51 and 56 by subsequent operation of the vcontrol device 55 in response to Waves from the path EA.

The operation of the system of Fig. 2 will now be described. It will be assumed that speech waves for transmission from west to east are being received over the two-Wire circuit L1, and, at that time, no speech waves for transmission from east to west are being received at station B over two-wire circuit Lz.

The speech Waves will be impressed by the hybrid coil H1 upon the input of the transmission path EA and transmitted thereover to the volume control device I. A portion of these speech Waves is diverted from the path EA in the input of the volume control device I by control circuit I 0, and another portion is diverted from the path EA in the output of the volume control device I by control circuit II, these two portions being impressed upon the control circuit 9 causing its operation in suitable manner to adjust the volume control device I in the path EA so as to maintain the speech waves in its output at a substantially constant level, and to adjust the compensating volume control device 8 in the path WA in inverse manner to insure that the transmission equivalent around the loop comprising paths EA and WA is not changed by the adjustment of the volume control device I. The speech waves in the output of the volume control device I are divided between the input of the modulator M1 and the input of the control circuit 41. The portion diverted into control circuit 41 after being delayed in the delay circuit 48 for a desired interval of time, which is preferably equal to T, is impressed upon the control device 49 causing its operation to energize the Winding of relay 50 in its output.

Relay 50 will then operate, and will be maintained operated as long as the speech waves are being substantially continuously impressed upon the control device 49 and for an additional hang-over interval preferably of T, to open and maintain opened the normally closed switch contacts 53 in the output of the control circuit 43, thereby preventing operation of the echo suppressor relays 45 and 46 due to subsequent operation of the control device 44 by the signal waves received from the path WA.

Meanwhile, the main portion of the speech waves impressed upon the input of the modulator M1 in the transmission path EA will be combined therein with the carrier frequency from the associated carrier frequency oscillator O1 to form combination modulated waves in the output of the modulator. Of these combination waves, one frequency band is selected by the band pass filter 2 and transmitted out over the path EA to the distant station B where it is selected by the band pass filter 3 and impressed upon the input of the demodulator DM1 at that station. The impressed waves are combined in the demodulator DM1 with the carrier frequency from oscillator O2 producing in the output of the demodulator a band of speech frequencies corresponding to the band impressed upon the modulator M1 at station A. This band in the output of the demodulator DM1 will be divided between the input of the compensating volume control device 4 in the transmission path EA` relation with each other and in energy-transmitting relation with the circuits L1 and L2 by means of hybrid coils H1, H2 and associated balancing networks N1 and N2.

The sender type terminal echo suppressor is located at station A and comprises a suppressor portion and an interlocler portion as in the case of the system of Fig. 1. The suppressor portion comprises the control circuit 65 having its input connected to the west to east transmission path EA, and comprising a delay circuit (i5, a signalcontrolled, relay-controlling device 5I and the windings of mechanical relays t8 and 69 connected to the output of the device 6l. The interlocker portion or" the terminal echo suppressor comprises a control circuit 'J9 having its input connected across the output of the path WA at station A, and comprising a delay circuit ll, a signal-controlled, relay-controlling device 'l2 and the Winding of mechanical relay 73 connected to the output of the device 12. Normally closed switch contacts lli are located in the transmission path WA at a point east of the input of control circuit l0, and are adapted to be opened to open-circuit the path WA by relay 6B when its winding is energized by operation of the control device G1 in response to the signal Waves received from the path EA.

The normally closed switch contacts l5 in the output of the control circuit 18 are adapted to be opened by the relay 68 when its Winding is energized by operation of the control device 6l in response to waves from the path EA, to prevent subsequent operation of the control device l2 by waves from the path WA from producing false operation of the relay 13.

The normally closed switch contacts 'it in the output of the control circuit are adapted to be opened by mechanical relay 13 when its winding is energized by operation of the control device 72 in response to Waves received from the path EA, to prevent subsequent operation of the control device El in response to Waves from the path EA from causing false operation of the relays 68 and 69.

As in the case of the receiver type of terminal echo suppressor illustrated in Fig. 2, the receiver type terminal echo suppressor located at station A in the system of Fig. 3 comprises a suppressor portion and an interlocker portion. The suppressor portion comprises a control circuit 'I'l having its input connected across the output of the path WA at a point therein between the connection thereto of the control circuit 'l0 and the hybrid coil I-Il, and comprising a signal-controlled, relay-controlling device 18 similar to the control device ll, and the Windings of mechanical relays 19 and 88 connected in parallel to the output of the control device 18. The interlocker portion of the receiver terminal echo suppressor comprises a control circuit Bl having its input connected across the input of the path EA between the point of connection of the control circuit 65 thereto and the hybrid coil H1 and comprising a delay circuit 82, a signal-controlled, relay-controlling device 83 and the winding of a mechanical relay 84 connected to the output of the device 83.

In the input of the path EA between the point of connection thereto of the input of control circuits 65 and BI are the normally closed contacts B5 adapted to be opened by relay 'i9 When its Winding is energized by operation or" the control device l8 in responseto Waves from the path WA, to prevent Waves subsequently received over the telephone circuit L1 from being transmitted to the telephone circuit L2 at the station B, or to the input of the control circuits 85 and 8l at station A. In the output of the control circuit 8l are the normally closed switch contacts 86 adapted to be opened by relay 8! when its Winding is energized by operation of the conu trol device 'S5 in response to Waves received from the path WA, to disable the input of the relay l. In the output ci' the control circuit 'll are the normally closed switch contacts 8l adapted to be opened by relay 86 when its Winding is energized by operation of the control device 83 in response uo Waves received from the path EA.

The delay circuits 65, ll and 82 are similar to the delay circuits shown in the systems oi Figs. l and 2. The delay circuit 66 in control circuit G5 is provided to give a delay in the operation o the suppressor portion of the sender terminal echo suppressor at station A, of preferably 2T, 2T being the overall transmission from the point in the path EA Where the input of control circuit 65 is connected thereto over the path EA through the hybrid coil Hz and over the return path WA to the point therein Where the disabling contacts 'Z4 are located. The delay circuit ll in control circuit I8 is provided to give a delay of preferably T in the operation of the interlocker portion of the sender type terminal echo suppressor located at station A. The delay circuit 82 in control circuit 8| is provided to give a delay of T in the operation or the interlocker portion of the receiver type echo suppressor at station A. These delays in the operation of the Suppressors might be obtained also by making the suppressor relays 58 and 63 and the interloeker relays 'F3 and 84 slow operating by suitable mechanical design, in which case the delay circuits 66, 'H and 82 would not be required.

The control circuit 65 controlling the disabling of the path WA and the disabling of the interlocker control circuit 70, the control circuit 'l controlling the disabling of the path EA, and the control circuit 8| controlling the disabling of the control circuit 1l, should, in the preferred case, that is, Where the delay circuits in these control circuits have the delay times specied above, have little or no hangover in their operation. This means that the suppressor relays 68, 19 and S8, and the interlocker relay 84 should be designed to release substantially instantaneously with cessation of supply of controlling currents to the control devices controlling their operation. In that case, the control circuit 'lll should have a hang-over in its operation of substantially T, which may be obtained, for example, by making the interlocker relay 13 sloW releasing to the required degree.

The operation of the system of Fig. 3 will now be described. It will be assumed that speech currents to be transmitted to station B are being received over the circuit L1 terminating at station A, and at that time noi speech currents to be transmitted from station B to station A are being received over the circuit L2 terminating at station B.

The speech currents from circuit L1 are impressed by hybrid coil Hi upon the input of the West to east transmission path EA and the main portion thereof Will be transmitted over that path to the station B Where it will be impressed by the hybrid coil H2 on the telephone circuit L2 over Which it will be transmitted to the cuit 8| after being 1 delayed` in transmission throughdelay circuit 82 for a vtime ofTwillbe impressed, upon the control device 33 causingits operation to venergize the windingr of relay S4 connected toits output Relayd willfoperate substantially instantaneously to open the normally-closed switchcontactsll in Lthe output of the controlcircuit l1 `forming the suppressor portion of` the] receiver. type terminal, echo suppressor` at station A, thereby preventing subsequentoperation'of control device .iii in re.- sponselto waves received over the vpath WA from; causing false `operation ofthe suppressor 4relays'li and 80. BecauseV of the delay of T introduced by the delay `circuit 82 in .control circuit 3 I, control circuitz'l'll will be rendered inoperative V substantially at the same time asthe main portionlvof `the signals transmitted `over thegpathiEA reaches station B. Meanwhile, the4 portion ofthe` we`st subscribers speech current diverted into control circuit 6 5-afte r beingdelayed in transmission` throughithe delay circuitlt therein for a time of, ,substantiallyI 2T will. be impressed on'` the control device, 6l .causingfits Yoperation to energizeilthe windings .gof relaysfi'! ,and 69. LRelay `|58 willoperate su' bstantially instantaneously` to open thenorinally closed switch contacts 'Min patipWA lrendering `tliatpath` .thereafter inop# elias. l currents `fr c`irn`the` station to the station B to transmitanyspeech `currents lor echo or `to the control circuit 7D rcontrolcirouit '1.1.` It AWillibe ,.seenrthat because of thend'elay of 2T caused. by delayfcircuit 66, inthe voperation of f thesuppressor relay `t8 fthe path WA Will remain l it openativeL to; Uransmit vto station Aor control circuits liliandsi'l untilthe time at which the echo currents due" to Qth'ejwest ,subscriblers speechEA current would be expectedto arrive 'at the disabliglg1 ypoint in.` thpath WA.y Relay 69 operates substantially simultaneously ,with relay G8 to open" normally V,closed contacts l5" in the output of the control circuit y'Hl thereby .pre-

l venting` subsequent operation of interlocker re`- lay T3 ,..by operation of control device` 'F2 due rto any speech currentswhich may have been stored in` delay `circuit TL prior to the openingjof switciifo'ontactsl in the path'WA.` A.

` The-westsubscribersspeechlcurrent nowfhas y gained complete control of ther echo" suppressor circuits associatedwith both sides of thefourwire circuitand will maintain this control `for atinie of sulostantially "23T afterxsaid subscriber has, ceased tallzingj due to the delay of 2T introducedM by `delay circuit (Sli `in control circuit ssgwntrhe ena or' an intervailof Tfafter the] west subscriber `has ceased talking, control gde?. vice 83 in control circuitmll releases causflug jre-` l'ay'lin its output to `fall.bao'lriretu-rniiig" switch v, contacts 8T in` controlcrcuit "Vt` to Itheir normal conditionu and., rendering` that circuit thereafter operative in condition vto be controlled bythe east subscribers speech currentff Atirne `after `-`control 'circuit ylfl has been rendered operative with, cessaucnjinme west subscribers, speech current.. control device 61 in control 'cir` 'andWBS inits output to fallback` returninglswitclieontacts 14 inthe path WA and the circuit `L1 over which it will be transmitted to the west subscriber. The portion of the east subscribers speech current divertedinto- G0111 trol circuit lil after being` delayed in transmissionV for a timeinterval ofuf while passing through the delay circuit 'H `will causcthe con troldevice `l2 to be operatedto venergizethe winding of interloclrer relay 13. i Interlockerrelay T3 will thenoperate to `open thenormallyj closed 4switch contacts `'lli disabling the echo suppressor controlled from the path EA.by..the west,V subscribers speech current. Another Vportion of the east subscribers `speech current will be diverted intov control `circuit 'Vl causing` the` operation of control devicei'ii` therein` to, enerf` gize thewindings of echo suppressor inlaysy "9- and 8D. Echo suppressor relay 'le willoperate toppen normally. closed switch contacts-Sinin the input of the path EA making that p21/.th ina operative toY transmit Ythe west `subscribers speech current to station B or to theinput of control circuits 65 and 8l., Relay im will opera ate simultaneously with relay- 'i9 tov open norl mally closed switch contactst in the output of control circuit `8L to disable the input of interlocker `relay M thuspreventing subsequent operation `of the control device 83 bythewest subscribers speech energy `transmitted to the control circuit 8l before switch contacts 357m the input` of `the path` EA have been opened,V l

from causng false operation of interloclrery re lay.84. g g

I fthe west subscriber` starts talking after they east subscriber starts talking` but within a time interval of T, it will be apparent lthat the portion of his speech current passing switch ccntacts 85.` in the input of the path EA before theseicontacts `have beeny opened by operation ofV echo suppressor relay 'i9 will be `transmitted over the `patlrEA to the. east subscriber associated with L2. kTheportion of these currents diverted into control circuit 8l, howevenfbecause of the delay introduced by delay circuitJ B22 therein, will not cause false operation of inter: locker relay 84 `because of the previous opera:-` tion of echo suppressor relay im" to block the, input of relay 8`4. The portion. of thewest subscriberis speech current diverted into control circuit 65,v because of the delay introduced thereby the delay circuitii6in control' circuit 'Eton' 60p in bydelay circuit `.66 willnot cause false operan Y the west subscribers speech current, it will be apparent that a portion of the east subscribers speech current will pass by the disabling point in transmission path WA before echo suppressor relay 68 under control of the west subscribers speech current has been operated to open the switch contacts 'I4 and thereby to disable the path WA. The main portion of the east subscribers speech current passing the disabling point, i. e., the point where the switch contacts 14 are located, in the path WA will be transmitted from vthe output of that path to hybrid coil H1 which will impress it upon the circuit Li over which it will be transmitted to the West subscriber.

A small portion of the east subscribers speech current passing the switch contacts 'Ill will be diverted into the control circuit 'l0 and after being delayed in the delay circuit 1| therein for a time interval of T, will be impressed upon the control device 'l2 causing 'its operation. The operation of control device 12, however, will not cause false operation of relay 13 to disable the control circuit 65 because echo suppressor relay 69 has been previously operated under control of the West subscribers speech current to disable the input of relay 13 by openingswitch contacts 15.

Another portion of the east subscribers speech current passing the switch contacts M in the path WA will be diverted into the control circuit 11 and will cause the operation of the control device 18 therein. The operation of the control device 18, however, will not cause false operation of echo suppressor relays 19 and 80 because relay 84 in control circuit 8| has been previously operated under control of the west subscribers speech current to disable the input of relays 'i9 and by opening switch contacts 81 in the output of control circuit 11. The hang-over of T in the interlocker relay 13 prevents false operation of echo suppressor relays 68 and 69 under control of the West subscribers speech current until after all of the east subscribers speech current has beentransmitted to the circuit L1.

In a modication of the circuit of Fig. 3, the delay circuit 56 in control circuit 65 would be designed to give a delay of T instead of 2T and the hang-over of echo suppressor relays 68 and 69 would be made substantially T instead of O and the delay in control circuit 'l0 would be substantially eliminated by omitting the delay circuit 1|.

It is apparent that in the system of Fig. 3, both of the echo Suppressors need not be located at one terminal as shown, but that the sender type echo suppressor may be located at one terminal station and the receiver type terminal echo suppressor located at the other terminal, or that either one may be located at a midpoint and the other located at either one of the terminals in which case the delays introduced into the control circuit and the hang-over times of the suppressor and interlocker relays may be somewhat diierent from the values indicated in Fig. 3. The echo suppressor system of Fig. 3 may, of course, be applied to a multiplex carrier telephone circuit similar to those shown in Figs. 1 and 2.

Although the invention has been described in connection with four-wire circuits employing continuous two-wire paths for transmission in either direction between terminal stations, it is apparent that the principles of the invention apply equally well to circuits employing short sections of two-wire lines for transmitting and' receiving at each terminal station, respectively,-

and employing as an intermediate link a long two-way transmission medium, such as the air link of a two-way radio telephone circuit or the submarine cable link of a two-way submarine cable telephone circuit. l

Although in the particular embodiments of the invention illustrated and described mechanical relays have been shown for conditioning the transmission paths by operating movable switches to close or open-circuit the paths, the invention is not limited to circuits employing the particular conditioning means shown. lFor example, within the scope of the invention the transmission paths ymay be effectively disabled by short-circuits or loss networks inserted in the paths, controlled by relays. Stationary devices having no movable elements may be employed in place of the mechanical relays shown, for example, vacuum tube relays, such as disclosed in Crisson Patent 1,647,212, issued November 1, 1926, or in the article by C. A. Beer and C. T. Evans in the Institute of P. O. Electrical Engineers (London), vol. 20, pp. 65 to 72 inclusive, published in 1920. v

What is claimed is:

l. In a two-way signal transmission systemv comprising at least near the terminals thereof normally operative one-way paths for trans-` mitting signals in opposite directions, means connected to the outgoing one-way path near at least one terminal of the system, and responsive to outgoing signals therein to disable the incoming path at the same terminal substantially at the same time as the arrival oi said outgoing signals at the other terminal of the system, and means connected to the incoming path near said one terminal, and responsive substantially immediately to incoming signals therein to disable the output of the incoming path-disabling means at the same terminal.

2. In a two-way signal transmission system comprising one-way paths for transmitting signals in opposite directions between distant stations, means at each station connected to the outgoing one-way path thereat and responsive to outgoing signals therein to disable effectively the incoming one-way path at the same station substantially at the same time as the arrival of said outgoing signals at the distant station, and means at each station connected to the incoming one-way path thereat yand responsive to incoming signals therein to disable substantially immediately the incoming path disabling means in its output at the same station.

3. In a two-way signal transmission system comprising normally operative one-Way paths for transmitting signals in opposite directions between widely separated stations, means at eachstation responsive to signals in the outgoing one-way path thereat to disable effectively the incoming one-Way path at the same station substantially at the time of arrival of said signals at the distant station, means at each station responsive substantially immediately to incoming signals in the incoming path thereat to disable the incoming path-disabling means at the same station in its output, and means at each station for maintaining said incoming path-disabling means disabled after cessation of said incoming signals thereat for a time substantially equal to the transmission time between stations over one of said one-Way paths. v 4l. In a two-way signal transmission system comprising normally operative one-way paths for transmitting signals in opposite directions between widely separated stations, means at each station responsive to signals inthe outgoing one-way path thereat to disable effectively the incoming one-way path at the same station substantially at the time of arrival of said signals at the distant station, means at each station responsive substantially immediately to incoming signals in the incoming path, thereat to disable the incoming path-disabling means at the same station, and means at each station for maintaining" the incoming path disabled after cessation of said outgoing signals for a time substantially equal to the transmission time between said stations over one of said one-Way paths. i

5. A two-way signal transmission system comprising a four-wire circuit connecting terminals and consisting of two oppositely-directed one- `way signal transmission paths which are normally operative, means at one'terminal only of said medium connected to the transmitting path thereat and responsive to signals therein to disable the `receiving path at said one terminal substantially at the time of arrival oi the transmitted signals at the distant terminal, means at said one terminal only connected to the receiving path thereat and responsive to signals therein to disable substantially immediately the transmitting path at said one terminal, means at said one terminal connected to the receiving path thereat and responsive to signals therein to disable substantially immediately the receiving path-disabling means at said one terminal in its output, means at said one terminal connected to the transmitting path thereat and responsive to signals therein to `disable the transmitting path-disabling means at said one terminal substantially at `the time of arrival of the transmitted signals at the distant terminal, and means at said one terminal responsive to operation `or" the transmitting path-disabling means thereat to prevent the disabling thereof thereafter in response to signals in the transmitting path at said one terminal.

6. In a two-way signaling system comprising at least near the terminals thereof one-way paths for signal transmission in opposite directions, means near at least one terminal of the system connected to the outgoing path thereat and responsive to outgoing signals therein for automatically controlling the amplification in said one-Way paths, means near said one terminal connected to the outgoing path thereat and responsive to outgoing signals therein to disable the incoming path at the same terminal, and means near said one terminal connected to the incoming path thereat, and responsive to incoming signals to disable the incoming pathdisabling means and said amplification-controlling means While maintaining said outgoing path operative to transmit outgoing signals.

7. In a two-way signal transmission system comprising at least near the terminals thereof normally operative one-way paths `for signal transmission in opposite directions, means near one terminal of the system connected -to theI outgoing path thereat and responsive to outgoing signals therein to automatically control the amplification in said one-way paths, means connected to the incoming path near said one terminal and responsive to incoming signals therein to disable the amplification-controlling means at the same terminal while allowing transmission of outgoing signals over said outgoing path thereat, means near said one terminal connected to the outgoing path thereat and responsive to outgoing signals therein to disable the incoming path and the amplicationcontrol disabling means at said one terminal, and means also responsive to the incoming signals in the incoming path near said one terminal when said incoming path is operative to disable the means responsive to outgoing signals for disabling said incoming path and said amplification control-disabling means.

8. In a two-way signal transmission system comprising at least near the terminals thereof normally operative one-way paths respectively for transmitting incoming and outgoing signals, means connected to the outgoing one-way path near at least one terminal of the system, and responsive to outgoing signals therein to disable the incoming path at the same terminal, means for delaying the disabling of said incoming path in response to said outgoing signals for a time interval substantially equal to the signal transmission time between the terminals of said system, and means connected to the output of the incoming path at said one terminal beyond the disabling point in that path, and responsive substantially immediately to incoming signals in said incoming path when it is operative, to disable said incoming path-disabling means in its output.

BJORN G. BJORNSON. 

